Commercial HVAC Troubleshooting: Diagnose 6 Common Issues Yourself
When your commercial HVAC system fails, you need answers fast: What’s wrong? Can we fix it? How much? When?
At Next Air, we’ve fielded thousands of emergency calls. We’ve learned: many issues can be diagnosed quickly by someone who knows what to look for.
This guide walks you through the 6 most common problems and when to call us.
At Next Air, we’ve fielded thousands of emergency calls. We’ve learned: many issues can be diagnosed quickly by someone who knows what to look for.
This guide walks you through the 6 most common problems and when to call us.
FAST FACTS
- 80% of issues are preventable with maintenance
- Most common issue: Clogged filter (DIY fix)
- Hardest to diagnose: Refrigerant leak
- Fastest fix: Filter replacement (5 minutes)
- Most expensive fix: Compressor replacement ($4,000-$8,000)
- Next Air Emergency Response: 1-2 hours, 24/7
Issue #1: System Won't Start/Turn On
Severity: High (system completely off) Most Likely Cause: 40% = Thermostat problem DIY Fix Possible: Yes, maybe
Step-by-Step Diagnosis
Step 1: Check the Thermostat (2 minutes)
Step 2: Check Electrical (3 minutes)
Step 3: Check for Safety Lockout (2 minutes)
- Is thermostat displaying normally?
- Is it set to HEAT (winter) or COOL (summer)?
- Is thermostat set to ON (not OFF)?
- What temperature is it set to?
- Verify thermostat is set to correct mode
- Ensure temperature setpoint is valid (68-72°F)
- Try adjusting temperature +/- 2 degrees
Step 2: Check Electrical (3 minutes)
- Walk to breaker box
- Look for any breaker switches in “OFF” or middle position
- Look for labels “HVAC,” “FURNACE,” “AC,” or “HEATING”
- If breaker is OFF: Flip it fully to ON
- Wait 2 minutes
- Try thermostat again
Step 3: Check for Safety Lockout (2 minutes)
- Look at thermostat screen for error codes (E1, E2, E3, etc.)
- E1 or “Lockout” = Safety feature activated
- E4 or “Sensor Error” = Thermostat sensor problem
- F0 or “Flame Failure” = Furnace ignition problem
- Note the error code
- Call us with the code: (587) 329-8603
- We can often diagnose over the phone
When to Call Us
Call immediately:- System locked out (error code)
- Burning smell
- Water damage
- Breaker tripping repeatedly
Issue #2: System Running But Not Cooling to Set Temperature
Severity: Medium Most Likely Cause: 40% clogged filter, 30% low refrigerant, 20% thermostat, 10% compressor DIY Fix Possible: Yes, partially
✗ If filter is clean: Proceed to next step
Step 2: Check Thermostat Settings (3 minutes)
Step 3: Check Outdoor Unit (5 minutes)
Go outside and inspect the condenser unit:
✓ What to do if blocked:
✗ If this doesn’t work: Proceed to next step
Step 4: Check System Operation (3 minutes)
Step-by-Step Diagnosis
Step 1: Check the Filter (5 minutes) This is the most common culprit.- Locate air return vent (large vent sucking air in)
- Look for filter rack or filter door
- Remove filter
- Hold filter up to light
- White/clean filter = Problem is something else
- Gray/dirty filter = FOUND IT!
- Note the filter size
- Purchase replacement filter (same size)
- Insert new filter
- Run system and wait 30 minutes
- Check temperature (should improve within 1-2 hours)
✗ If filter is clean: Proceed to next step
Step 2: Check Thermostat Settings (3 minutes)
- Is it set to COOL mode?
- Is FAN set to AUTO (not OFF)?
- What is temperature setpoint? (Should be 74-78°F)
- Verify COOL mode is selected
- Set fan to AUTO
- Set temperature to 72°F
- Wait 15 minutes and check if system responds
Step 3: Check Outdoor Unit (5 minutes)
Go outside and inspect the condenser unit:
- Is it running? (You should hear fan spinning)
- Is there debris around it? (Leaves, dirt, branches)
- Is there ice/frost on it?
- Can you see the condenser fins clearly?
✓ What to do if blocked:
- Clear leaves and debris
- Use garden hose to gently rinse fins (low pressure)
- Wait 30 minutes
- Check temperature again
✗ If this doesn’t work: Proceed to next step
Step 4: Check System Operation (3 minutes)
- Listen to system for 30 seconds
- Do you hear compressor humming?
- Is condenser fan spinning?
- Are air vents pushing cold air?
- Possible low refrigerant (leak)
- Possible electrical issue
- Cost to fix: $400-$2,000
- Possible low refrigerant, dirty coils, thermostat miscalibration
- Cost to fix: $300-$1,200
When to Call Us
Call within 24 hours:- Filter was clean but system still not cooling
- Compressor not running
- System running but only modest improvement
Issue #3: Strange Noises
Severity: Varies (could be minor or major) Most Likely Cause: 30% loose components, 30% bearing/fan, 20% compressor, 20% other DIY Fix Possible: No (stop operation immediately) Cost Range: $100-$5,000
What it sounds like: Shaking/metallic rattling Most likely cause: Loose bolt, bracket, or panel vibration Risk level: Low What to do: Stop system, call us (within 24 hours) Cost to fix: $100-$300
Noise #2: Squealing/High-Pitched Noise
What it sounds like: High-frequency squeal or squeak Most likely cause: Fan motor bearing OR belt problem Risk level: Medium (can fail suddenly) What to do: Stop system immediately, call us ASAP (same-day preferred) Cost to fix: $300-$800 (bearing or belt replacement)
Noise #3: Grinding Sound
What it sounds like: Deep grinding, mechanical grinding Most likely cause: Compressor bearing failure Risk level: HIGH – EMERGENCY What to do: STOP IMMEDIATELY, call emergency line TODAY Cost to fix: $3,000-$8,000 (compressor replacement) Why: Compressor is most expensive part. Continuing operation worsens damage.
Noise #4: Hissing Sound
What it sounds like: Continuous or intermittent hissing Most likely cause: Refrigerant leak Risk level: Medium-High What to do: Stop system, call within 24 hours Cost to fix: $500-$2,500 (find leak, seal, recharge)
Noise Types & Diagnosis
Noise #1: Rattling SoundWhat it sounds like: Shaking/metallic rattling Most likely cause: Loose bolt, bracket, or panel vibration Risk level: Low What to do: Stop system, call us (within 24 hours) Cost to fix: $100-$300
Noise #2: Squealing/High-Pitched Noise
What it sounds like: High-frequency squeal or squeak Most likely cause: Fan motor bearing OR belt problem Risk level: Medium (can fail suddenly) What to do: Stop system immediately, call us ASAP (same-day preferred) Cost to fix: $300-$800 (bearing or belt replacement)
Noise #3: Grinding Sound
What it sounds like: Deep grinding, mechanical grinding Most likely cause: Compressor bearing failure Risk level: HIGH – EMERGENCY What to do: STOP IMMEDIATELY, call emergency line TODAY Cost to fix: $3,000-$8,000 (compressor replacement) Why: Compressor is most expensive part. Continuing operation worsens damage.
Noise #4: Hissing Sound
What it sounds like: Continuous or intermittent hissing Most likely cause: Refrigerant leak Risk level: Medium-High What to do: Stop system, call within 24 hours Cost to fix: $500-$2,500 (find leak, seal, recharge)
Issue #4: Unusually High Energy Bills
Severity: Medium (financial, not safety) Most Likely Cause: Clogged filter (40%), thermostat (20%), low refrigerant (20%), aging system (20%) DIY Fix Possible: Yes, partially
Clogged filter forces system to work 30-50% harder.
Follow instructions in Issue #2 above (filter diagnosis)
Cost to fix: $15-$40 Expected savings: $100-$200/month
Step 3: Check Thermostat Settings (2 minutes)
Step 4: Request Professional Energy Audit (30 minutes)
If filter and settings are fine, request professional energy audit:
Step-by-Step Diagnosis
Step 1: Compare to Previous Years (2 minutes)- Last June = $800 electric bill?
- This June = $1,200?
- 20%+ increase = investigate
Clogged filter forces system to work 30-50% harder.
Follow instructions in Issue #2 above (filter diagnosis)
Cost to fix: $15-$40 Expected savings: $100-$200/month
Step 3: Check Thermostat Settings (2 minutes)
- Is thermostat set to reasonable temperature? (68-70°F winter)
- Is someone overriding settings daily?
Step 4: Request Professional Energy Audit (30 minutes)
If filter and settings are fine, request professional energy audit:
- System efficiency test
- Refrigerant charge check
- Ductwork inspection
- Coil condition assessment
When to Call Us
Call if:- Energy bills jumped 20%+ year-over-year
- Filter is clean but bills still high
- System is 10+ years old (efficiency decline normal)
Issue #5: System Cycles On/Off Repeatedly
Severity: Medium Most Likely Cause: Low refrigerant (40%), thermostat (30%), electrical (20%), blockage (10%) DIY Fix Possible: No Cost Range: $300-$1,500
Problem Cycling: Runs 2-3 min, stops 1 min, repeats constantly
Cost to fix: $300-$1,500 How urgent: Within 24 hours
What This Means
Normal Cycling: Runs 15-20 min, stops 10 min, repeats smoothlyProblem Cycling: Runs 2-3 min, stops 1 min, repeats constantly
Diagnosis
Step 1: Check thermostat settings (same as Issue #2) Step 2: Check filter (same as Issue #2) Step 3: Call us for refrigerant checkCost to fix: $300-$1,500 How urgent: Within 24 hours
Issue #6: Water Pooling Under Indoor Unit
Severity: HIGH (mold risk) Most Likely Cause: Condensate drain blockage (90%) DIY Fix Possible: Yes, partially Cost Range: $0-$300
Step 3: DIY Clearing (10 minutes – if comfortable)
Emergency First Aid
IMMEDIATE ACTION:- Turn off system
- Place bucket under drip
- Call us within 4 hours (prevent mold)
Step-by-Step Diagnosis & Fix
Step 1: Locate the Drain (2 minutes)- Look under furnace/air handler
- You should see small PVC pipe going down
- Is water flowing through drain?
- Or pooling in the trap?
Step 3: DIY Clearing (10 minutes – if comfortable)
- Get small wire or pipe cleaner
- Gently poke into drain opening
- Try to break up blockage (usually algae/mold)
- Run water from nearby tap through drain
- Water should flow freely now
When to Call Us
Call within 4 hours if:- Drain won’t clear
- Water continues pooling
- You can’t locate the drain
Get Professional Help
Your troubleshooting helped narrow it down. Now let our certified technicians take it from here.
CALL NOW FOR DIAGNOSIS: (587) 329-8603 EMERGENCY SERVICE: 24/7 Available (1-2 hour response)
CALL NOW FOR DIAGNOSIS: (587) 329-8603 EMERGENCY SERVICE: 24/7 Available (1-2 hour response)